The present invention relates to a variable-frequency oscillation system for determining an output frequency of a power converter apparatus employing inverters.
In a case where inverters are operated in synchronization with a commercial power source or where a plurality of inverters are operated in parallel relative to a commercial power source, it is required to use an oscillator having a frequency variable function capable of detecting the phase difference between the power source and the inverters and controlling the frequency for about several %.
As satisfying such requirement, a variable-frequency oscillator using a crystal oscillator has been known and a prior art example is described referring to FIG. 1 for an understanding of this invention.
FIG. 1 is a circuit diagram in block form showing the prior art example wherein the output of a crystal oscillator 11 is introduced by way of a F/V converter (frequency-voltage converter) 12 to an adder 13. A control voltage is also applied by way of another path to the adder 13 and the output from the adder 13 is branched into a portion to be introduced by way of a PI controller (proportional integration type controller) 14 to a V/F converter (voltage-frequency converter) 15, the output signal of which is taken out by way of a frequency divider 17 as an output frequency and into another portion to be fed back by way of a F/V converter 16 to the adder 13.
Referring now to the operation, clock pulses of frequency fcry from the crystal oscillator 11 are converted in the F/V converter 12 into a signal Vcry and introduced into the adder 13. While on the other hand, a control voltage Vc is applied to the adder 13 and the sum of the input value Vcry and the control voltage Vc form a setting value for determining the output frequency.
The output from the adder 13 is converted by way of the PI converter 14 and the V/F converter 15 into an output signal N.times.fo, a portion of which is converted by way of the frequency divider 17 into an output signal f0 at 1/N frequency and the other portion of which is converted by way of the F/V converter 16 into a voltage V.sub.N .times.fo and fed back to the adder 13.
Accordingly, the output from the adder 13 is the difference between Vc+Vcry and V.sub.N .times.fo and, by controlling this difference to zero in the PI controller 14, the output frequency N.times.fo from the V/F converter 15 is a frequency that is determined by fcry and the control voltage Vc, provided that the gain and the linearity are the same for the V/F converter 15 and the F/V converter 16.
Since the accuracy for the final output frequency fo is determined by the accuracy of the F/V converters 12, 16 in the prior example, it is required for the converters 12, 16 that they have the same gain and the linearity and are highly accurate and stable. Particularly, in a case of a CVCF inverter (constant voltage constant frequency inverter), since very high accuracy is demanded for the output frequency, it is required to restrict the fluctuations including temperature changes, aging changes and the likes to less than 0.1%, whereby higher accuracy and stability are required for the F/V converters.
However, since F/V converter is generally composed by using resistors, capacitors, diodes, differential amplifiers and the likes, circuit structure is complicated very much and a lot of controlling sections are required as well, in order to satisfy the foregoing performances.
This results in drawbacks of impairing the reliability and the stability, making the entire apparatus larger and increasing the cost thereof.
In addition, since the output frequency N.times.fo from the V/F converter is generally used while divided into a lower frequency by the frequency divider, a V/F converter operated at a frequency as high as several MHz is required for the digital control or the like, thereby making the converter itself very much expensive.